It is known in the art relating to engine emission control that valve event induced combustion residuals in the cylinders are a fundamental limitation on permissible intake/exhaust valve overlap for providing acceptable engine idle smoothness and stability. Many sophisticated methods have been employed by various manufacturers to overcome this limitation. These include intake valve event phasing, exhaust valve event phasing, and dual independent camshaft phasing.
Unfortunately these solutions are not readily applicable to single overhead camshaft (SOHC) or cam-in-block engines because the exhaust and intake cam lobes are on the same physical camshaft. A camshaft phaser, or cam phaser, controlling such a cam/valve train has only a very minor impact on trapped idle residuals, and generally increases residuals as the timing is advanced or retarded significantly from an optimum timing without variable timing capability, which worsens the idle quality situation.